A Hidden Words puzzle aimed at 11- to 14- year olds, containing mathematical words that they should know

Instructions below

There are 32 hidden words hidden in this grid. They are all mathematical words that you should know for Key Stage 3 (UK). They appear horizontally - left or right, vertically - up or down, and diagonally - forwards or backwards. All words have four or more letters.
At the beginning you do not know which words to look
for! But if you want to make it easier, click the box next to "Words
Visible" and the words will be revealed. [Try not to do this - try to
find some words before you look at the list.]
When you notice a word, left-click on the first letter of the word,
drag the mouse to the end of the word, and then release the mouse button.

If you were right, the letters on the grid will change colour, and
the word will appear on the right of the screen in that same colour.
You may see a word (eg "ANGLE") that doesn't work. This may be because
it is part of a longer word (eg "TRIANGLE"). You must find the
whole word.
When you have found all 32 words, the letters that haven't been used
will spell out an important rule about angles (and its nickname).
Keep going until you've found all 32 words. It's quite hard, but
have fun!